The invention relates generally to an apparatus and method for shaping beam patterns (the acoustic response) of sonar systems. In particular, the apparatus and method provide a low cost technique for shading an array of elements with as few as two elements, which is particularly effective but not limited to shallow water sonar applications.
The goal of a sonar system is to receive energy from a wide field of view, while still being able to reject energy from certain undesired angles. As illustrated in FIG. 1, the sonar system 10 should receive energy from the wide field of view 12 illustrated, but be able to reject energy received from the surface, in the form of surface clutter 14 from the water surface 16. The classical approach for achieving this objective is to use an array of elements (or hydrophones) and a beamformer, as illustrated in FIG. 2.
The processing of multichannel data arises naturally when manipulating data from an array of spatially distributed sensors. The problem of coherently summing the outputs from such a collection of sensors is known as beamforming. A beamformer permits one to listen preferentially to wave fronts propagating from one direction over another. With the addition of a filter on the output of each sensor prior to the summation as shown in FIG. 2, conventional beamformers can provide both spatial and spectral filtering on the incoming wave field.
As illustrated in FIG. 2, a sound source 20 emits signals in the look direction of the sonar system 10. The sound source typically is an echo signal from a target that has been ensonified. The sonar system 10 also receives noise in the direction of the sound source 20, as well as noise 22 from directions other than the look direction. The signal from sound source 20 and noise 22 are received by an array of spatially distributed sensors 24 including sensor.sub.0, sensor.sub.1, . . . , sensor.sub.N-1. The output of sensor.sub.0 through sensor.sub.N-1 are input to band pass filters 26, amplified by amplifier 28, and input to conventional beamformer 30.
The beam pattern produced by such a linear array has side lobes, wherein energy from unwanted directions is received. The classic approach for correcting for the formation of side lobes is aperture shading. When low side lobes are required, amplitude shading of array elements is used to reduce the side lobes by reducing the amplitude contribution of some of the elements.
The traditional approach for performing amplitude shading includes subdividing the linear array aperture and applying symmetric shading coefficients to each of the elements. Using this approach requires a minimum of three elements in the linear array in the plane where the beam pattern is desired.
An example of a conventional linear array is shown in FIGS. 3(a) and 3(b). The array 32 of FIG. 3(a) includes seven isotropic elements 34 spaced .lambda..sub.0 /2 apart along a straight line, where .lambda..sub.0 is the wavelength of the center frequency .omega..sub.0 of the array 32. The received signals are summed to produce an array output signal. The beam pattern, that is, the relative sensitivity of response to signals from various directions, is plotted in a plane over an angular range of -.pi./2&lt;.theta.&lt;.pi./2 for frequency .omega..sub.0. This pattern is symmetric about .theta.=0.degree. as well as .theta.=90.degree., and the main lobe 36 is centered at .theta.=0.degree.. The largest-amplitude sidelobe 38, at .theta.=24.degree., has a maximum sensitivity which is only 12.5 dB below the maximum main-lobe sensitivity. The same array configuration is shown in FIG. 3(b); however, in this case the output of each element is delayed in time by delays 40, before being summed. The resulting directivity pattern now has its main lobe 36 at an angle of .psi. radians, where: ##EQU1## where .omega..sub.0 =the normalized frequency of received signal (in radians)
.lambda..sub.0 =the wavelength at frequency .omega..sub.0 (in meters) PA1 .delta.=the time-delay difference between neighboring element outputs (in number of samples-seconds) PA1 d=the spacing between antenna elements (in meters) PA1 c=the signal propagation velocity equal to (.lambda..sub.0 .omega..sub.0)/(2.pi.T) in meters/second, and PA1 T=the time step delay (in seconds).
The sensitivity is maximum at the angle .psi. because signals received from a plane-wave source incident at this angle, and delayed as in FIG. 3(b), are in phase with one another and produce the maximum output signal. For the example illustrated, d=.lambda..sub.0 /2, .delta.=(0.8131/.omega..sub.0), and therefore .psi.=sin.sup.-1 (.delta..omega..sub.0 /.pi.)=15.degree..
There are many possible configurations for phased arrays of elements. FIGS. 4(a) and 4(b) show one such configuration where each of the array element outputs is weighted by two weights in parallel, one being-preceded by a time delay of a quarter of a cycle at frequency .omega..sub.0 (i.e., a 90.degree. phase shift, or .pi.T/2.omega..sub.0 seconds.sup.2). The output signal is the sum of all weighted signals, and since all weights in weighting circuits 42 are set to unit values, the beam pattern at frequency .omega..sub.0 is by symmetry the same as that of FIG. 3(a). For purposes of illustration, a directional sinusoidal noise 44 of frequency .omega..sub.0 incident on the array is shown in FIG. 4(a). The angle of incidence (45.degree.) of the noise 44 is such that it would be received on one of the sidelobes 46 of the beam pattern with a sensitivity only 17 dB less than that of the main lobe 36 at .theta.=0.degree..
If the weights 42 are symmetrically set as set forth below, the beam pattern at frequency .omega..sub.0 is modified: ##STR1##
In this case, the main lobe 36 is almost unchanged from that shown in FIGS. 3(a) and 4(a), while the particular sidelobe 46 that previously intercepted the sinusoidal noise 44 in FIG. 4(a) has been shifted so that a null is now placed in the direction of the sinusoidal noise 44. The sensitivity in the noise direction is 77 dB below the main-lobe sensitivity, improving the noise rejection by 60 dB.
Each element in the linear array 32 requires electric wiring and cabling to connect the element to the beamformer 30. Further, as the number of elements is increased, the number of inputs to the beamformer 30 also increases, and as a result, the beamforming operation becomes more complex. In order to perform beamforming on an array with a large number of elements, a large amount of computer processing time is required.
The present invention achieves a beam pattern, which is substantially equivalent to a beam pattern achieved with a large number of elements, utilizing only two elements. The use of only two elements permits the array to be extremely simple because wiring and cabling for only two elements is required, and only two inputs are supplied to the beamformer, thereby requiring less computer processing time to perform the beamforming operation.
The present invention is an improvement over conventional apparatus and methods for performing shading on wide beam sonar systems, in that the present method and apparatus only requires two elements. Further, the outputs of these two elements can be passively summed or combined into one amplifier. Still further, the measured pattern demonstrates the cutoff rate and rejection of energy towards unwanted angles, such as in the direction of surface clutter.